In many of the magnetic disk memories now in use, the read/write transducer employs a ferrite core for the magnetic flux path. As is well known, such a core includes a short read/write flux gap at which point the flux in the core flows through the medium. The structure of these cores typically comprises a so-called "C" core piece and an "I" core piece which are bonded together with a thin layer of non-magnetic material in the read/write gap area to provide the spacing needed to form the gap. Tyically, read/write gap length is held to a few tens of microinches. Such a small length permits close packing of the individual bits linearly along each data track on the disk.
Individual cores are formed by a process which starts with the formation of a relatively long C core piece bar (C bar) and a relatively long I core piece bar (I bar). The cross section of each is uniform and identical to that of an individual C core piece or I core piece respectively. The two bars are bonded together in the configuration forming a cross section identical to that desired for a complete core using a thin layer of solder glass sputtered on the gap faces. The two bars are pressed together and heated to melt the solder glass and create the bond. After machining the transducing face to set gap height, the composite bar is then sliced transversely into the individual cores.
To allow interchangeability of written disks among individual transducing heads, it is important to control read/write gap length during the manufacturing process, so that each head has a gap length within a preselected tolerance. In one manufacturing method, this is accomplished by sputtering a preselected thickness of alumina on the bar faces defining the gap before sputtering on the solder glass. When the two pieces are then bonded together, only the small amount of glass bonding material remains between them, and the thickness of the alumina layer(s) and the glass bonding material form the entire gap length. Typically, the glass bonding layer is very thin relative to the length of the alumina layer, so that accurate control of the alumina layer thickness accurately defines gap length.
In the described core construction, where the I core piece is bonded to the two ends of the C core piece, in effect two flux gaps are created. The desire to reduce the reluctance within individual cores throughout the flux path except at the read/write flux gap, makes it desirable to keep the length of the gap not used for reading and writing, the so-called back gap, as short as possible. In effect, it should have merely the length of the bonding glass layer thickness. Therefore, during the alumina sputtering operation, it is necessary to shield the back gap face areas in some way to prevent alumina from attaching itself in those areas and undesirably increasing the reluctance of the magnetic path through the cores ultimately created.
This shielding has in the past been accomplished in one process through the use of mechanical shields simply placed on the back gap areas of the I and C core pieces during the sputtering step. When dealing with large numbers of these magnetic bars in a single sputtering operation as is usually the case, this requires a large number of these shields, all of which must be individually placed to shield the bars. Apart from the fact that this is time consuming and hence expensive, it is also possible that these shields will be accidently moved when the tray carrying the cores is placed in the sputtering oven, which if not corrected results in unusable parts. Another approach employs deposited masks to shield these areas. While reliable, this requires additional steps of first depositing and then removing the masks, and hence adds cost to the overall process. Accordingly, there is substantial motivation for finding a simple, effective, inexpensive means for providing this shielding.